There are a small number of possible reasons why your bill would be higher than anticipated.

1. It may be your first invoice from us, which is likely to be prorated
2. Tariff or bolt-on allowances have been exceeded in the month, without spending controls being in place
3. Services have been accessed that are outside your inclusive tariff allowances

In more detail:

1. Prorated Invoice

Our billing follows a calendar monthly cycle. Therefore, if you became a plan.com customer part way through a month, your first invoice will include line rental for the calendar month in advance, plus the days of the month you connected that you had not yet been invoiced for.

2. Allowances Exceeded

Your tariff includes standard UK calls, standard UK texts (SMS) and UK/EU mobile data allowances. For example, you may have a 5Gb allowance of mobile data to use in the calendar month. If that allowance is exceeded and you have not taken advantage of our spending controls, the additional usage will be charged at our standard rates. https://www.plan.com/pricing

3. Non-Inclusive Usage

As above, our mobile tariffs come with unlimited calls to standard* UK landlines and mobiles that are in the UK, together with unlimited text messages (SMS) to standard* UK mobiles that are in the UK.  If you have called a number or accessed a service that falls outside these parameters, you will have been charged for doing so.

*Standard means calls to 01, 02, 03, 07 numbers and the freephone numbers, 0800 and 0808. 'Standard' does not include premium rate and non-geographical numbers. MMS are not included as they are not text messages (SMS).

Gold & Platinum account users can manage spending through the CONTROLS option in My.Plan. Upgrades are available for Essential account users

To configure spending controls, from your My.Plan dashboard, go to: CONTROLS > SPEND CONTROLS 

Choose from the sub-options:

• Data Limits (UK/EU and or Roaming)
• Calls Limits (UK/EU and or Roaming)
• Text Limits (UK/EU and or Roaming)

For each sub-option, select the user(s) to which the restrictions should apply, and select SET UP (or  VIEW, if an existing spending control is to be reconfigured). Select from the options presented and SAVE your choices. 

Bulk configurations can be managed through the MANAGE ALL DEVICES option in the usage type column header 

For example, to be notified when 'User 1' has reached 80% and 100% of their UK/EU mobile data allowance and set an absolute limit of £10 of additional mobile data usage, the pathway would be:

CONTROLS > SPEND CONTROLS > DATA LIMITS >UK/EU Data > [User 1] > SET UP> [Notification: tick 80% and 100%] > [Set Spending Limit At £10 of additional charges] > [Accept Terms] > SAVE

See also How To Configure a Profile